1. Field of the Invention
This invention relates generally to portal imaging devices for radiation treatment systems, and more particularly to imaging devices for linear accelerators (linacs) used in radiation therapy.
2. Description of the Prior Art
The use of a linear accelerators in radiation therapy is generally known and is described e.g. in the brochure "A primer on Theory and Operation of Linear Accelerators in Radiation Therapy", U.S. Department of Commerce, National Technical Information Service December 1981. Such linear accelerators are used for treating patients with X-rays or electron beams. Such X-rays are created when high energy electrons are decelerated in a target material such as tungsten. Alternatively, the electrons themselves may be used directly for treatment.
The major modules in a linear accelerator are a gantry with treatment head, a stand, a control console and a treatment couch. The stand is anchored firmly to the floor and the gantry rotates on bearings in the stand. The operational accelerator structure, housed in the gantry, rotates about a horizontal axis fixed by the stand for treatment of a patient laying on the treatment couch.
In the radiation therapy treatment of a patient, geometric accuracy is a very important factor to the success of the treatment. From the article "Use of Digital Fluoroscopy as an On-line Verification Device in Radiation Therapy" by Joseph Leong in Phys. Med. Biol., 1986, Vol. 31, No. 9, 985-992, several methods for local control of the delivery of radiation are known. These methods are portal verification films and record-and-verify systems.
Portal verification films do not deliver images of sufficient quality in certain situations, particularly when high energy X-rays are used in the treatment and they need a relatively long time to develop. Furthermore, films are expensive and the images can only be seen after the treatment dose has been delivered.
A known record-and-verify system includes an imaging section for delivering an image using a fluoroscopic technique and an image processing section for digitally processing the image. In the imaging section a fluorescent screen converts images generated by the X-rays, which are emitted from the treatment head and then pass through the patient, into visible images. The visible images are then reflected to a video camera by a reflector in order to avoid irradiating the camera. In the image processing section the video signals from the camera are digitally processed in real-time for continuous monitoring of the treatment field throughout the treatment.
Known portal imaging devices have been enclosed in rigid light tight boxes. In order for the boxes to cover a reasonable radiation field size, the construction of the detector enclosure has to be very bulky, which poses an inconvenience during patient set-up and occupies space in the treatment room when not being used. Practical use of such devices is thus limited.